1. Field of the Invention
The present invention relates to an amorphous transparent conductive thin film with a low internal stress and a low resistivity used in solar batteries, display elements such as liquid crystal display (LCD) elements, organic electroluminescence (EL) elements, inorganic EL elements, and touch panels, an oxide sintered body used as a raw material when manufacturing the transparent conductive thin film by a DC sputtering method, and a sputtering target which uses the oxide sintered body.
2. Description of the Related Art
A transparent conductive thin film has a high conductivity (for example, a resistivity of 1×10−3 ohm-cm or less) and a high transmittance in the visible light range. Therefore, besides being used in electrodes in solar batteries, liquid crystal display elements, and other kinds of light-receiving elements, it is also used as a heat reflecting film for automobile windows and building windows, as various kinds of antistatic films, and in anti-fog transparent heating elements for refrigerated showcases.
For the transparent conductive thin film, tin oxide (SnO2) films doped with antimony or fluorine, zinc oxide (ZnO) films doped with aluminum or gallium, and indium oxide (In2O3) films doped with tin, and the like, are widely used. In particular, the indium oxide film doped with tin, that is to say an In2O3—Sn film, called ITO (indium tin oxide) film, is often used because of the ease in which a transparent conductive thin film with a low resistivity can be obtained.
As a method of manufacturing these transparent conductive thin films, the sputtering method is often used. The sputtering method is an effective technique in a case where a material with a low vapor pressure is used to form a film on a film formation substrate material (hereunder, simply referred to as “substrate”), or when precise film thickness control is needed, and is widely used because of the high degree of convenience of its operation.
In the sputtering method, it is common to use a raw material containing the desired component of the film as a target. In this method, a vacuum apparatus is generally used, and after the vacuum vessel containing the arranged target and substrate has been brought to a high vacuum, a noble gas, such as argon, is introduced, and under a gas pressure of approximately 10 Pa or less, the substrate is made the anode, the target is made the cathode, and argon plasma is generated by initiating a glow discharge between them. The positive argon ions in the plasma are thus made to collide with the target, which is the cathode. As a result, the particles of the target component are expelled, and the particles are made to accumulate on the substrate, forming a film.
The sputtering method is classified according to the method of generating the argon plasma. A method using radio-frequency plasma is called a radio-frequency sputtering method, and a method using DC plasma is called a DC plasma sputtering method. In contrast to the radio-frequency sputtering method where it is possible to form a film even with a nonconductive target, in the DC sputtering method it is necessary to use a conductive target. However, generally, the DC sputtering method has, a faster film formation speed compared to the radio-frequency sputtering method, inexpensive power-supply equipment, and the film formation operation is simple, and for these reasons, it is widely used industrially.
The film formation speed of sputtering is closely related to the chemical bonding of the target material. Sputtering is a phenomenon that occurs when positive argon ions having kinetic energy collide with the target surface, and the target surface matter which receives the energy is expelled. Therefore, the weaker the interionic bonding or interatomic bonding of the target material, the higher the probability of expulsion by sputtering.
The electrodes for LCDs, organic EL elements, and the like, require a transparent conductive thin film with a smooth surface. In particular, in the case of an electrode for an organic EL element, a high surface smoothness is demanded of the transparent conductive thin film, because super thin films of organic compounds are formed on top of them. Surface smoothness is, generally, greatly affected by the crystallinity of the film. Even with the same composition, a transparent conductive thin film (amorphous film) with an amorphous structure in which there are no grain boundaries, has a more favorable surface smoothness compared to a transparent conductive thin film (crystalline film) with a crystalline structure.
Even in the case of an ITO film with a conventional composition, the amorphous ITO film obtained by decreasing the substrate temperature at the time of film formation when performing sputtering film formation at low temperature (150 degree.C. or less) and at high gas pressure (1 Pa or more), has a superior surface smoothness. The limit of the resistivity of the amorphous ITO film is 9×10−4 ohm-cm, and to form a film with a small surface resistance it is necessary for the film itself to be formed thick. However, when the film thickness of the ITO film becomes thick, a problem of coloring arises.
Furthermore, even regarding an ITO film in which film formation occurred at room temperature without heating of the substrate, if the sputtering gas pressure is low, the kinetic energy of the sputter particles injected into the substrate is high, locally increasing the temperature, and resulting in a film formed by minute crystal phases and amorphous phases. The presence of minute crystal phases can be confirmed by, as well as X-ray diffraction, transmission electron microscopes and electron diffraction.
When such minute crystalline phases have formed in one part, it causes a large effect on the surface smoothness. Furthermore, upon etching removal of the transparent conductive thin film to a predetermined shape by a weak acid, there is a problem in that only the crystalline phase is unable to be removed and remains in some instances.
On the other hand, the amorphous ITO film has, as well as a problem of resistivity, a problem of stability. In a situation where an amorphous ITO film is used as an electrode for LCDs, or organic EL elements, or the like, in the manufacturing process, heating to above 150 degree.C. is performed after electrode formation, and the transparent conductive thin film crystallizes as a result. The reason for this is that the amorphous phase is a metastable phase. If the amorphous phase crystallizes, a crystal grain is formed, and a problem arises in that the surface smoothness worsens, and at the same time there is large change in the resistivity.
Next, the organic EL element is described. The EL element uses electroluminescence. It has a high visibility because of self-luminescence, and is a completely solid-state element. As a result, the EL element has advantages such as superior shock resistance, and the use of EL elements as light emission elements in various types of display devices is receiving much attention.
For EL elements, inorganic EL elements using inorganic compounds as the luminescent material, and organic EL elements using organic compounds exist. Among these, because the organic EL elements are able to greatly lower the drive voltage and are easy to miniaturize, they are being aggressively researched for practical use as the next generation of display elements. An organic EL element has a basic configuration of an anode/luminescent layer/cathode lamination, and a configuration where a transparent anode is formed on a substrate using a glass plate and the like, is normally employed. In this case, the luminescence is taken out from the substrate side.
Incidentally, in recent years, for the following reasons, attempts have been made to take out the luminescence from the cathode side by making the cathode transparent. By making the anode transparent along with the cathode, an entirely transparent light emission element can be made. Accordingly, as a background color of the transparent light emission element, an arbitrary color can be employed, also allowing it to be a colorful display when not luminescent, thus improving its decorativeness. Furthermore, in a case where black is employed as the background color, there is an advantage in that the contrast improves at the time of luminescence. Moreover, it is possible to use color filters and color conversion layers, and to place these on top of the light emission element. As a result, the light emission element can be produced without consideration for color filters and color conversion layers. Accordingly, the anode can be formed separately to the color filters and color conversion layers which are inferior in heat resistance. Therefore it is possible to raise the substrate temperature when forming the anode, and as a result the value of resistance of the anode can be lowered.
By making the cathode transparent, such advantages can be obtained. Therefore production of organic EL elements using transparent cathodes is being trialed.
For example, the organic EL element described in Japanese Patent Application Publication No. H10-162959, consist of a structure where an organic layer containing an organic luminescent layer lies between the anode and cathode, the cathode is constructed by an electron injecting metal layer and an amorphous transparent conductive layer, and the electron injecting metal layer touches the organic layer.
Furthermore, in Japanese Patent Application Publication No. 2001-43980 there is disclosed an organic EL element devised to efficiently take out the light from the cathode by making the cathode transparent and using an optically reflecting metal film for the anode.
Next, each layer in the structure of the organic EL element is described. Firstly, regarding the electron injecting metal layer, this is a metallic layer which is able to satisfactorily inject electrons into the organic layer which contains the luminous layer. To obtain a transparent light emission element, it is favorable for the electron injecting metallic layer to have a light transmittance of at least 50%. Therefore, it is necessary for the film thickness of the layer to be a super thin film of approximately 0.5 nm to 20 nm.
Specifically, the electron injecting metallic layer includes films with a film thickness of 1 nm to 20 nm using a metal (electron injecting metal) with a work function of 3.8 eV or less, for example, Mg, Ca, Ba, Sr, Li, Yb, Eu, Y and Sc. In this case, a structure is desired where a light transmittance of at least 50%, and preferably 60% or more is obtained.
The organic layer which lies between the anode and the cathode contains at least, a luminous layer. The organic layer may consist of only a luminous layer, or it may be a multilayered construction where, as well a luminous layer, a hole injection transportation layer and the like, is laminated. In the organic EL element, the organic layer has functions such as; (1) a function where at the time of application of an electric field, it is able to receive an injection of holes from the anode or hole transportation layer, as well as being able to receive an injection of electrons from the electron injecting layer, (2) a transportation function which moves the injected electric charges (electrons and holes) by the force of the electric field, and (3) a luminescence function to provide a place inside the luminous layer where the electrons and holes recombine, linking this to the luminescence.
The hole injection transportation layer is a layer consisting of a hole conduction compound, and has a function to transmit holes injected from the anode to the luminous layer. By placing the hole injection transportation layer between the anode and the luminous layer, more holes are injected to the luminous layer under a lower electric field. Furthermore, the electrons injected to the luminous layer from the electron injection layer accumulate near the interface inside the luminous layer due to the barrier of electrons present at the interface between the luminous layer and the hole injection transportation layer. As a result, the luminous efficiency of the organic EL element can be improved, and an organic EL element with superior light emission performance can be obtained.
Next, the anode is described. The anode is not particularly restricted provided it exhibits a work function of at least 4.4 eV, and preferably a conductivity of at least 4.8 eV. A metal with a work function of at least 4.8 eV, or a transparent conductive thin film, or a combination of these, is preferable.
It is not necessary for the anode to be transparent in all cases, and it may be coated with a black carbon layer, or the like. Examples of suitable metals include Au, Pt, Ni and Pd. Furthermore, examples of conducting oxides include In—Zn—O, In—Sn—O, ZnO—Al, and Zn—Sn—O. Examples of laminated bodies include an Au and In—Zn—O laminated body, a Pt and In—Zn—O laminated body, and an In—Sn—O and Pt laminated body.
Furthermore, it is acceptable if the interface between the organic layer and the anode has a work function of at least 4.4 eV. Therefore the anode can be double-layered, and a conductive film with a work function of 4.4 eV or less can be used on the side which does not touch the organic layer. In this case, metals such as Al, Ta and W, and alloys such as Al alloys, and Ta—W alloys can be used. Furthermore, conducting polymers such as doped polyaniline and doped polyphenylene vinylene, and amorphous semiconductors such as a-Si, a-SiC, and a-C can also be used. In addition, black semiconducting oxides such as Cr2O3, Pr2O5, NiO, Mn2O5, and MnO2 may be used.
Next, the cathode is described. It is desirable for the transparent conductive layer which constitutes the cathode of the organic EL element to be an amorphous film with a small internal stress and a superior smoothness. Furthermore, it is preferable for the resistivity value to be 9×10−4 ohm-cm or less, in order to remove voltage drop and nonuniformity of luminescence caused thereby.
To realize a transparent conductive thin film with a superior surface smoothness, and which is stable even under the heat history of the production process, is impossible with conventional ITO materials, and accordingly, it is difficult to use these in transparent electrodes in display elements such as organic EL displays and LCDs.
As an amorphous film, zinc added indium oxide has been described in Japanese Patent Application Publication No. H7-235219. This publication introduces where the Zn element content is 10 at % to 20 at % with respect to the total amount of the Zn element and the In element, and a stable amorphous nature and a high conductivity is shown.
However, the film with the composition introduced here had a shortcoming in that the optical transparency at shorter wavelengths of visible light, in particular at wavelengths near 400 nm, was low.
Furthermore, an indium oxide thin film containing tungsten at a W/In atomic number ratio of 0.004 to 0.047 is described in Japanese Patent Application Publication No. 2004-52102, and shows stability in obtaining an amorphous film, and a high conductivity. However, in manufacture by the sputtering method, there is a shortcoming in that it is difficult to obtain a film with an internal stress with an absolute value of 1×1010 dyn/cm2 or less (that is to say −1×1010 dyn/cm2 to 1×1010 dyn/cm2).
Moreover, if productivity and the reduction of production costs are considered, there is a necessity to employ the DC sputtering method, and to perform high-speed film formation using a high DC power. However, it is becoming understood that depending on the additional elements to the sputtering target for manufacturing an indium oxide thin film, if a high DC power is applied, arcing can occur in some cases, making it impossible for high-speed film formation. If arcing occurs at the time of film formation, it becomes a cause of a generation source of particles, which causes a decrease in product yield. If arcing occurs continuously, formation of the film itself is hindered.
Furthermore, as the sputtering target is used in sputtering, it gradually becomes a state where it is pitted in parts. However, the fact that it can be used with its sputtering characteristics constant up until just before being all used up, is useful from the aspect of material costs. However, depending on the sputtering target, as the integrated value of the applied power increases, nodules (black protrusions on the target surface) occur on the surface of the sputtering target, causing problems such as the occurrence of arcing, and a decrease in the film formation speed.
For a sputtering target for which arcing occurs on a small scale, arcing can be avoided by using a power supply having an arcing control function. As a method for controlling arcing, there is the DC pulsing method (method of neutralizing the charge (electrostatic charge) on the target by periodically stopping the negative voltage applied to the target and applying a low positive voltage during that time), and there is a method of installing an arc-blocking circuit (a circuit that detects an increase in the discharge current when arcing occurs, and stops the power supply before it can grow into full arcing, then restarts the power supply after the current flowing to the target drops sufficiently) (refer to “Transparent Conductive Film Technology”, Ohmsha, pg. 193 to 195).
However, a power supply having such arcing control functions is very expensive, thus increasing equipment cost. Furthermore, even when a power supply having such arcing control functions is used, it does not mean that arcing can be completely controlled.
[Patent Document 1] Japanese Patent Application Publication No. H10-162959.
[Patent Document 2] Japanese Patent Application Publication No. 2001-43980.
[Patent Document 3] Japanese Patent Application Publication No. H7-235219.
[Patent Document 4] Japanese Patent Application Publication No. 2004-52102.
[Non Patent Document 1] “Transparent Conductive Film Technology”, Ohmsha, pg. 82, pg. 193 to 195.